How to build the simplest FM transmitter?

This tiny transmitter has several "defects": smaller radius of the service area, lower quality of the sounds and the relatively unstable frequency. These can be considered as a compromise to easily have your own transmitter for the time being or as a more positive choice. These "defects" are only from the perspective of conventional transmission such as "clear stereo sound to receive anywhere". Artist could change these to another directions. Whether or not, you can experience a convivial wireless imagination by this transmitter.

This has a condenser microphone unit in it. Every version could use the same circuit. Usual microphone has not enough output to this unit, but condenser microphone unit can drive this system. See how simply it is installed--->picture

Q: The transmitter works but the airwaves are too weak. I think there might be something wrong.

A: Every transmitter using the minimum parts like this has to have harmonic signals and "spurious". So you may sometimes not receive the authentic signal. Leave your radio receiver far at least three meters from the transmitter. Don't attach any cable to the antenna terminal. Then turn the trimcap slowly. The frequency will change and you will find the most strongest signal. (See "the another Q&A below) In the 1.1 version with a variable capacitor to change the frequency, turn it very slowly until you get the most strongest signal at your radio. If you don't get, reduce the turn of the coil from 4 upto 2. When you use a variable capacitor over 10PF such as 20, 50 PF, 4 turns (I indicate in the schematic) would be too much. If you have a frequency counter, it would be easy to tune up the coil.

Q: I love the simplest transmitter. I wonder if this transmitter could be used for a practical Mini FM transmission.

A: The main problem is that the frequency is unstable. This is interesting from the perspective of radio performance, though. As the voltage of the battery is deteriorated, the transmitting frequency will be changed. In order to avoid it, you may use a stabilized power supply. It would be a bit expensiver. You don't mix with a "AC adapter". When you use a ordinary "AC adapter", you should add a voltage regulator semiconductor for 9-12 volts such as "7809" or "7812". The simplest schematic is here (jpg).

Q: This transmitter can cover only a modest radius. Is it possible to extend it?

A: If you connect it to the proper antenna, you can do it. How to make an antenna is here (pdf). For this, you have to use a thicker coaxial cable between the transmitter and the antenna. The thinest one can be used only for 1-2 meter extension. Think of the thickness of waterpipe and the length. Long extension increases the loss.

A: The schematic is supposed to use copper coated plate that is familiar with my design.
When it is difficult to obtain, you can use even "porous circuit board" (jpg) that is popular in electronic hobby. The schematic is here (gif). I suggest you use the reverse side and directly solder the parts to it because this is convenient for your possible cut-and-paste and would be effective for your electro-magnetic performance.

A: In order to change the transmitting frequency without the trimcap, you a bit spread or compress the gap of the coil. When you do this operation, you have to have some distance (at least 3 meter) between the transmitter and the receiver (FM radio): otherwise you may be disturbed by many "spurious" (harmonic signals) and may not find the main one you want. That's why the best thing is to use a frequency counter.

A: Don't mix it with "variable capacitance diode (varicap)" for the 1-3 W transmitter. What we need here is only for manually varying the capacitance from (theoretically) 0 to 20 PF max. There are the various types and appearances. The classical one consists of tiny brass plates and ceramics. Some of the pictures are here.

A: There should be many replacements: 2SC2003, 2SC458, 2SC1973, 2SC3358, 2SC3580, 2SC3605, 2SD734 and so on. The European or American models such as BC337 or NTE123AP can be used, but be careful of the different position of the leads (E=Emitter, C=Collector and B=Base) depending on the semiconductors. The circuit that I designed uses 2SC2001 because it is cheap and very obtainable in Tokyo. See an example layout for BC337

A: I stopped showing the schematic with it because the tunable mold coil (3-4 turns) has become less obtainable now. If you can get it, just install it instead of a hand-made coil WITHOUT the trimmer capacitor.

A: You can get a "condenser microphone unit" in low price. Using this, you can build it very easily. Check the partial diagram (pdf) and
the complete diagram(pdf) of the simplest transmitter with a microphone in it. When this is acrobatically soldered, here(jpg) is the example.

Q: When I connect my iPod to the audio input of this transmitter, it suddenly stops to work. What's happening?

A: The ealier model of iPod has an automatic shut-down system in it. When you pull out the plug of your headphone, it automatically stops working. When the left and right channels are short-circuited, the same thing happens. Presumably, you use a stereo cable and solderd the left (white) and right (red) lead together. Please use the left channel lead (white) only. Usually the left output is compatible with mono audio. If you, however, insist to mix the left and right output of your audio source into the transmitter, make a simple "interface" like this by two sets of 1 kiro ohm registers.